Accurate loss measurements of power system apparatus, such as large power transformers, high voltage reactors, power capacitor banks, insulator bushings, and power cables are difficult due to the low per factor conditions during testing. Of all these measurements, the most critical is the measurement of losses of large power transformers. There is a penalty, which can be as high as $10,000/kW for no-load and $5,000/kW for load loses, for every kilowatt of loss exceeding the guaranteed value. The economic impact of uncertainties associated with the measurement of high-voltage power losses is very high. Therefore traceability and the acceptable accuracy limits of high-voltage power measurements are becoming increasingly more important and critical to manufacturers and utilities. It is important that high-voltage power measuring systems be calibrated after installation and recalibrated on a regular basis to maintain their quoted accuracy and to ensure traceability to higher echelon standards. Manufacturers are now being required to provide documentation certifying the accuracy of their high-voltage power measuring systems through a calibration process that is traceable to such standards.
The manner in which a calibration circuit is used as a load loss standard is explained in a paper by P. N. Miljanic et al. entitled "An Improved Current-Comparator-Based 1000-A Transconductance Amplifier for the In-Situ Calibration of Transformer Loss Measuring Systems" published in IEEE Trans. Power Delivery, vol. 8, pp. 861-865, July 1993. In particular this paper shows in FIG. 1 how a load loss standard is connected to a load loss measuring system under test.
U.S. Pat. No. 4,795,969 to Eddy So issued Jan. 3, 1989 discloses the use of an improved current-comparator technique for obtaining a load loss standard for in-situ calibration of a load loss measuring system. More specifically, this patent provides an active voltage divider and unity-gain integrator for generating reference voltage signals E.sub.0 and E.sub.90 that are respectively in-phase and in quadrature with a test voltage source E.sub.H. The signal E.sub.0 is applied to an adjustable voltage divider. The reference signals E.sub.0 and E.sub.90 are applied through reference resistors to a current comparator in the forms of in-phase and quadrature reference current signals I.sub.0 and I.sub.90 proportional to E.sub.0 and E.sub.90 respectively. The signals E.sub.0 and E.sub.90 are also supplied to an amplifier assembly (including a summing amplifier and a transconductance amplifier), that generates a standard load current I.sub.L that is supplied to the current comparator through the loss measuring system under test. As more fully explained in the So patent, the primary function of the current comparator is to correct for errors.
The phase of the standard load current I.sub.L can be adjusted by the voltage divider which varies the magnitude of the in-phase voltage E.sub.0 relative to a fixed magnitude for the quadrature voltage E.sub.90. This phase adjustability is required in order to test the loss measuring system at different power factors. However, the effect of varying the voltage E.sub.0, while the voltage E.sub.90 remains unchanged, is to vary the value of their summation voltage, and hence the value of the load current I.sub.L. As a result, adjustment of the voltage divider not only varies the phase of the output current I.sub.L but also its magnitude, which is an undesirable side effect.
Another disadvantage of the prior system is that there is no provision for changing the level of the output current without changing the test voltage.